Electrophoretic display devices are one example of a bi-stable display technology, which use the movement of charged particles controlled by an electric field to provide a selective light scattering or absorption function.
In one example, white particles are suspended in an absorptive liquid, and the electric field is used to bring the particles to the surface of the device. In this position they may perform a light scattering function so that the display appears white. Movement away from the top surface enables the color of the liquid, for example black, to be seen. In another example, there may be two types of particles, for example black negatively charged particles and white positively charged particles, suspended in a transparent fluid.
Electrophoretic display devices enable low power consumption as a result of their bi-stability (an image can be retained with no voltage applied), and they enable thin and bright display devices as there is no need for a backlight or polarizer.
Another important factor in electrophoretic displays is low-cost manufacturing. Since the devices can be made from plastic materials, cost reduction by means of reel-to-reel manufacturing is possible. For such manufacturing, it is desirable that the electronic devices only require the use of a single display medium layer. Low cost requirements further makes it desirable to employ passive direct drive addressing schemes. The most simple configuration of such a display device is a segmented reflective display. A segmented reflective electrophoretic display has low power consumption, good brightness and is bi-stable in operation, and thus able to display information even when the power is turned off.
A known electrophoretic display in the form of a passive matrix and utilizing particles having a threshold, comprises a lower electrode layer, a display medium layer accommodating the particles suspended in a transparent or colored fluid, and an upper electrode layer. Biasing voltages are applied selectively to electrodes in the upper and/or lower electrode layers to control the state of the portion(s) of the display medium associated with the electrode being biased.
One type of electrophoretic display devices uses so-called “in-plane switching”. In-plane electrophoretic displays use selective lateral movement of the particles in the display material layer. When the particles are randomly dispersed, they block the passage of light to the underlying surface and the particle color is seen. The particles may be colored and the underlying surface black or white, or else the particles can be black or white, and the underlying surface colored.
Advantages of in-plane switching are that the device can be adapted for both reflective and/or transmissive operation. Particles may be moved to create a passageway for light, so that both reflective and transmissive properties can be implemented. This enables operation by illumination using a backlight as well as reflective operation by illumination with ambient light and using a reflector. The in-plane electrodes are provided on one substrate, or two substrates facing each other provided with electrodes. In-plane electrophoretic display devices can provide viewing angle independent brightness and color.
Active matrix addressing schemes are also used for electrophoretic displays. These are generally required when fast image updating is desired for bright full color displays and higher contrast ratio with many grayscales. Such devices are of particular interest for signage and billboard applications, and as (pixelated) light sources in electronic window and ambient lighting applications. Colors are implemented using color filters, and the display pixels then function as grayscale devices, or by a subtractive color principle, or a combination using both color filters and a subtractive color principle.
Known electrophoretic displays are driven by complex driving signals. For a pixel to be switched from one gray-level to another, it is often first switched to white or black in a reset phase and then to the final gray-level. Gray-level to gray-level transitions and black/white to gray-level transitions are more slower and more complicated than black to white, white to black, gray to white or gray to black transitions.
For example, in a color display a particle with a characteristic absorption spectrum used, and the display may be driven in such a way that many different levels of absorption at the characteristic wavelengths can be achieved. By accurate control of the number of magenta particles, for example, the optical density of the medium in the green part of the spectrum can have many different values, also known as “gray levels”.
One significant problem with electrophoretic cells, in particular when these are arranged in a passive matrix based display, is the time taken to address and write an image. This is partly owing to that the pixel output is dependent on the physical position of the particles within the pixel cells, and that moving the particles over a certain distance takes time, and since the particles in general move relatively slow, also writing of the electrophoretic cells is slow. This is a particular problem in IPEP displays where the particles typically have to move over larger (in-plane) distances than in the case of “out-of-plane” devices.
Typical pixel addressing/writing times range between several tens to hundreds of milliseconds for small-sized pixels in out-of-plane switching electrophoretic displays and up to several minutes for larger-sized pixels in in-plane electrophoretic displays. Furthermore, the displacement speed of the particles scales with the applied field. Thus, in principle, the higher the applied field, the shorter the addressing/writing time, however, high voltages to generate these field are not always available or possible to use.
In its simplest form an electrophoretic pixel can be controlled to exhibit two different colors, e.g. black and white, i.e. 1 bit color. This is unacceptable for e-books and e-signage, which are considered to require at least 4-6 bit grayscales. At present, the number of accurate and reproducible gray-scales that can be achieved in commercially available products is just 4.
Reproducing more than 1 bit color, in a reproducible and uniform manner, and for more than one type of pigment is therefore desirable. This, however, requires control of more than one type of particles per cell, which in turn requires more complex cells, e.g. cells comprising more electrodes, or multiple cells of different colors to form multi-color pixels. In one known example multi-color is achieved by stacking two or more monochromatic display layers of different color. However, due to the stacking the image quality is impaired, or parallax may arise and/or the light absorption is increased.
Another aspect in in-plane electrophoretic display devices is the pixel aperture, i.e. the part of the cell area that directly contributes to the visible output, which comprises parameters such as contrast, brightness, viewing angle, color saturation, parallax, Moiré etc. It is desirable that the visible area of the pixel is as large as possible. However, since the number of particles in a pixel is constant, the pigments that result in a dark state must be stored somewhere during a bright state. This is achieved at the expense of the pixel's aperture by means of storage electrodes.
US 20040239613 discloses an electrophoretic display device including a back substrate that is spaced apart from and facing an observation side substrate. The cells are located in the space between the substrates, which space is filled with a transparent insulating migration liquid in which there are two types of charged particles. The particles differ in charge polarity and coloration. In each cell the display area is in the form of transparent display electrode that is disposed on one of the substrates and there are two collection electrodes, one at each substrate and facing each other. In US 20040239613 the two types of particles are moved both laterally and vertically with respect to the planes of the substrates. In order to accomplish a multicolor display, US 20040239613 discloses stacking of the cells where each cell in the stack has differently colored particles. Since a display electrode covers the visible area of the pixel, this inevitable leads to increased parallax, Moiré and increased light absorption and thus less bright pixels. It also leads to more complex and more expensive devices.